Evaluation of the protective efficacy of Ornithodoros moubata midgut membrane antigens selected using omics and in silico prediction algorithms

Abstract

The African argasid tick Ornithodoros moubata transmits two important pathogens, the African swine fever virus and the spirochete Borrelia duttoni, the cause of human relapsing fever. To date, only conventional control measures such as widespread application of acaricides, strict control measures, and animal movement restrictions have been implemented to confine these diseases. Vaccines against tick infestations have the potential to be among the most efficacious interventions for the management of these diseases. Plasma membrane-associated proteins upregulated in tick midgut cells in response to blood feeding and digestion are thought to play vital functions in tick physiology and in the transmission of tick-borne pathogens. In addition, their antigenic extracellular regions are easily accessible to antibodies synthesised by immunised hosts, which makes them interesting targets for tick vaccine design. The mialomes (midgut transcriptomes and proteomes) of unfed O. moubata females and of engorged females at 48 h post-feeding have recently been obtained, providing a wealth of predicted midgut protein sequences. In the current study, these mialomes were screened using in silico tools to select predicted antigenic transmembrane proteins that were upregulated after feeding (516 proteins). The functionally annotatable proteins from this list (396 proteins) were then manually inspected following additional criteria in order to select a finite and easy-manageable number of candidate antigens for tick vaccine design. The extracellular antigenic regions of five of these candidates were obtained either as truncated recombinant proteins or as KLH-conjugated synthetic peptides, formulated in Freund's adjuvant, and individually administered to rabbits to assess their immunogenicity and protective potential against infestations by O. moubata and the Iberian species Ornithodoros erraticus. All candidates were highly immunogenic, but provided low protection against the O. moubata infestations (ranging from 7% to 39%). Interestingly, all candidates except one also protected against infestations by O. erraticus, achieving higher efficacies against this species (from 20% to 66%). According to their protective potential, three of the five antigens tested (Om17, Om86 and OM99) were considered little suitable for use in tick vaccines, while the other two (OM85 and OM03) were considered useful antigens for tick vaccine development, deserving further studies.

In silico selection of functionally important proteins from the mialome of Ornithodoros erraticus ticks and assessment of their protective efficacy as vaccine targets

“…The anti-OeRPP0 response induced significant protection (57.5%) to O. erraticus by reducing nymph moulting and female oviposition and fertility without any protective effect to O. moubata, despite the strong recognition of its RPP0 orthologue. This higher sensitivity of O. erraticus vs O. moubata to vaccines based on midgut antigens has already been observed and discussed by our team in previous studies, where it was reasoned that there may be particular factors in O. moubata (anatomical, physiological, molecular or microbial) that decrease the accessibility of immune effectors to their targets thus reducing the vaccine efficacy of the midgut concealed antigens against this species [20–22]. The protection provided by OeRPP0 to O. erraticus was lower than the 90–96% reached with peptide pP0 to ixodid ticks by [42, 43], which consisted in reduced feeding performance, reduced moulting and reproduction, and increased mortality.…”

“…Once titrated, the immune sera were reacted in ELISA and western blot to tick saliva and the four midgut protein extracts (soluble and membrane proteins from fed and unfed females) from O. erraticus and O. moubata [22]. The ELISA plates were coated with 1 µg of saliva or midgut extract per well, the sera were diluted 1/300 in TPBS and the PO-anti-rabbit IgG was diluted 1/10,000.…”

“…Accordingly, midgut proteins involved in these processes may be interesting targets for developing vaccines aimed to control ticks and tick-borne pathogens. In fact, in our previous studies with Ornithodoros ticks we have observed that plasma membrane-associated proteins from tick enterocytes induce protective immune responses in vaccinated animals [20–22].…”

Cocktail Anti-Tick Vaccines: The Unforeseen Constraints and Approaches toward Enhanced Efficacies

“…For these reasons, alternative approaches to tick control have been suggested [ 18 ], of which vaccination or immunological control is regarded the most promising, environmentally friendly, and sustainable strategy. To date, numerous antigens have been reported to induce protection against Ixodid ticks [ 18 , 19 , 20 , 21 ] and less against Argasids [ 22 , 23 , 24 ]. Earlier, Willadsen [ 25 ] questioned whether combining antigens toward enhanced efficacy is a valid hypothesis.…”

“…Certainly, there are reasons for this slow progress which will not be addressed in this review. Therefore, it is not coincidental that to date only a few single vaccines against Argasids have been reported [ 22 , 23 , 24 ]. Nonetheless, tick researchers still aim to develop good vaccines which are able to induce a substantial humoral/or cell mediated protective immune responses in Argasid and Ixodid ticks.…”